I. Field of the Invention
The present invention relates to wireless communication. More particularly, the present invention relates to method and apparatus for concurrently processing multiple calls in a spread spectrum communications system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communication in which a large number of system users are present. Although other multiple access communication system techniques are known in the art, such as time division multiple access (e.g., TDMA and GSM), frequency division multiple access (FDMA), and AM modulation schemes such as amplitude companded single sideband (ACSSB), the spread spectrum modulation technique of CDMA has significant advantages over these other modulation techniques for multiple access communications systems. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” issued Feb. 13, 1990, and U.S. Pat. No. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” issued Apr. 7, 1992, both assigned to the assignee of the present invention and incorporated herein by reference.
CDMA systems are typically designed to conform to one or more particular CDMA standards. Examples of such CDMA standards include the “TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” “TIA/EIA/IS-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (collectively, the IS-95 standard), the TL/EIA/IS-98-A, -B, and -C standards entitled “Recommended Minimum Performance Standard for Dual-Mode Spread Spectrum Cellular and PCS Mobile Stations,” and “The cdma2000 standards for spread spectrum systems,” (hereinafter, the IS-2000 standard). New standards are continually proposed and adopted for use.
Each of the standards noted above defines a mechanism for processing a single call between a mobile station and a base station. The mechanism is characterized by a call processing state machine on the signaling layer (i.e., layer-3) that includes a number of states and a set of allowed transitions between the states. Each state in the state machine corresponds to a particular state of the mobile station (or base station) with respect to the call being processed. A transition to a new state takes place upon the occurrence of certain specified events.
CDMA systems are originally designed to (primarily) provide voice communication. Consequently, the call processing state machine defined by the CDMA standards is designed to support a single call, which is typically a voice call. For systems that conform to a particular CDMA standard and designed to implement the call processing state machine defined by that standard, only one call can typically be processed at any given moment, and a new call cannot be processed until the active call is terminated. This one-call limitation restricts the type of services that can be provided to the user.
As modern day communication evolves, it is highly desirable to provide enhanced communications services beyond just voice-only or data-only communication. These enhanced services often rely on the ability of the system to concurrently support multiple calls. For example, the ability to concurrently transmit voice and video (e.g., via two concurrent calls) can be used to provide video conferencing. For some applications, it is desirable to allow for concurrent transmission of voice and data (e.g., transfer of a file while carrying on a conversation).
Thus, techniques that allow for the concurrent processing of multiple calls in a spread spectrum environment are highly desirable.